The present invention relates to a broad band phase shift system, and more particularly to a broad band system using reflective means.
Phase shift systems are well known in the art. They are in general characterized by axes of principal polarizations and an angle of phase shift or retardation angle associated with these axes. They may be of two types: transmissive or reflective. One example of a transmissive phase shift system is a common quarter-wave quartz plate. In the quarter-wave quartz plate, electromagnetic radiation impinging on the plate will be transmitted therethrough. However, a 90.degree. phase retardation would occur between the principal polarizations. By broad band phase shift system it is meant that the same degree of phase retardation would occur over a wide range of frequency of incident beam. Thus, a 90.degree. broad band transmissive phase shifter would retard the phase by 90.degree. between the principal polarizations for a wide range of incident electromagnetic beams having differing frequencies.
A quartz plate, however, is not a broad band phase shifter. Typically, a ten percent (10%) bandwidth phase shifter, i.e. a phase shifter operative over the range of V.+-.0.05 V (where V is frequency), is considered broad band.
Reflective phase shifters are also known in the art. In Applied Optics, Volume 19, page 2688 (1980), and Volume 18, page 1875 (1979), the author Southwell describes a reflective phase shifter using multi-layer dielectric material. Similarly, in Journal of Optical Society of America, Volume 69, page 1437 (1979), M. L. Scott also describes a 90.degree. reflective phase shifter, constructed of dielectric multi-layer coatings. However, in both Scott and Southwell (1979) articles, the reflective phase retarder constructed was of narrow bandwidth.